PROJECT SUMMARY (Project 1) The mainstay therapy for castration-resistant prostate cancer (CRPC) consists of agents targeting the androgen receptor (AR) signaling pathway and chemotherapies, which induce antitumor responses, and in select men a vaccine and a radiopharmaceutical bone-targeting therapy. The responses with all of these therapies tend to be short-lived. Conversely, treatment with the cytotoxic T-lymphocyte antigen 4 (CTLA-4)? targeting drug, ipilimumab, which promotes enhanced T-cell responses and generates memory immune responses, has led to durable regression of metastatic disease and an overall survival benefit. A recent phase 3 trial of ipilimumab in patients with CRPC demonstrated dramatic responses in a subset of them and improved survival in those with favorable clinical characteristics, who also tend to have good responses to therapies targeting the AR signaling pathway. Previous studies demonstrated that inhibition of AR signaling has positive effects on the immune system. We hypothesize that patients with cancers that regress with therapies targeting AR signaling will benefit from the addition of anti-CTLA-4 (ipilimumab) immunotherapy. In addition, given the lack of imaging capabilities to differentiate between tumor growth and tumor infiltration by immune cells as a result of immunotherapy, we propose to develop radiolabeled antibody imaging to evaluate tumor responses to immunotherapy. We aim to rationally integrate anti-CTLA-4 (ipilimumab) immunotherapy with agents targeting the AR signaling pathway to provide durable clinical benefit with improved survival in patients with prostate cancer, and utilize novel imaging techniques to accurately identify tumor responses. In this project, we will identify molecular changes associated with clinical outcomes of anti-CTLA-4 (ipilimumab) immunotherapy for prostate cancer by examining matched tumor and peripheral blood specimens obtained in recently completed clinical trials. We will perform in-depth examination of the specimens using immunohistochemistry, immunofluorescence, flow cytometry, gene expression studies, and serum cytokine analyses. We will prospectively evaluate the most promising molecular determinants in our novel clinical trial with the aim of linking the interactions between the immune system and the AR signaling pathway. Finally, because our group has identified other immunologic molecules that may be targeted with radiolabeled antibodies for imaging, we propose to evaluate this possibility in murine models to provide data for future clinical trials. Our proposed studies will provide data to enable integration of anti-CTLA-4 (ipilimumab) immunotherapy into a treatment strategy for prostate cancer that induces durable responses and improves overall survival, with the goal of curing the disease.